The present invention relates to a method and apparatus for determining shear elasticity and shear viscosity of a material, and more particularly to a method for determining the shear elasticity and shear viscosity of a material based on resonance spectra of the medium under test.
The study of objects in terms of their mechanical response to external forces is of considerable interest in material science and medical diagnosis. Changes of elasticity of soft tissues are often related to pathology, and therefore the study of and characterization of changes in elasticity of materials can be an important diagnostic tool.
Traditionally, the mechanical characteristics of tissue have been examined through palpation. Palpation is a process in which a static force is applied to tissue and an estimation of the tissue elasticity is made through the sense of touch. While providing some information regarding the characteristics of the tissue, this method is highly dependent on the opinion of the medical practitioner estimating the force, and, although often useful, is not repeatable and does not provide a useful scale for characterizing the tissue.
Another prior art method for characterizing the mechanical properties of tissue is elasticity imaging, which has been the subject of extensive investigation in recent years. Elasticity imaging provides a quantitative method for measuring the mechanical properties of tissue. Generally, an excitation force is applied to the tissue and the response of the tissue is used to reconstruct the elastic parameters of the tissue. These parameters are typically related to the shear modulus, or “hardness” of the tissues being imaged. While providing a means for repeatably characterizing tissue, however, the ability of conventional B mode ultrasound imaging to differentiate various tissues depends principally on the acoustic impedance, which in turn depends upon the bulk modulus of the tissue under examination. The range of variation of bulk modulus, however, is relatively small. Therefore, the bulk modulus does not vary sufficiently as a function of the state of the tissue to allow for a characterization of the tissue.
Recently, vibro-acoustography, a method that can image the “hardness” of an object, has been developed. In vibro-acoustography, a confocal transducer having a center disk and an outer ring introduces two ultrasound beams to the same focal spot in an object. The two ultrasound beams have slightly different frequencies: for example, 1.001 MHz, and 0.999 MHz. At the focal spot, the interference of these two beams causes the object to vibrate at the beat frequency, in this example, at 2 kHz. Acoustic emissions from the object are detected by an acoustic hydrophone. These emissions contain information about the local material properties of the object.
By scanning the focal plane of the transducer in a raster manner, a 2D image of the object can be generated. In this method, the applied force is oscillative, allowing the dynamic properties of the material to be examined. The force is also confined to a local spot, therefore providing good spatial resolution of the image. This method is therefore particularly useful in detecting hard inclusions in soft material. For example, it has been used to image calcification in human arteries, microcalcification in breast tissue, and fractures in metal parts.
In vibro-acoustography, the brightness of a pixel is related to the stiffness and reflectivity of that location. However, the image is not a direct representation of a single elastic modulus. Rather, it combines information about several material properties of the object. Therefore, present versions of vibro-acoustography do not provide a direct evaluation of the stiffness of a material under examination.